Cortisol is a glucocorticoid steroid produced by the adrenal cortex in response to ACTH.
Mechanism: The body's primary stress hormone. Mobilizes glucose, suppresses immunity, modulates blood pressure, and follows a strong diurnal rhythm. Peak ~30 min after waking, trough at night. Acute spikes are protective; chronic elevation is destructive.
Lab range: AM serum 6–23 µg/dL; salivary 4-point curve is more useful for HPA function than a single serum draw.
Caveats: A single morning serum is nearly useless for diagnosing HPA dysfunction. Use a 4-point salivary panel (AM, midday, evening, bedtime) to assess rhythm. Chronically flat cortisol = HPA dysregulation, not 'adrenal fatigue' (which isn't a recognized diagnosis).